I don't know about laws over there... but over here I take a full "buyout/rights transfer" as meaning they now technically own the images and I have no stake in them anymore. It also means, they do not need to let me show the images or say I shot them anymore.
Very few clients actually need this. Most of my clients I steer towards an "Exclusive" license where I cannot resell the images. Often there are other stipulations, but I retain rights control still and they feel happy that they got the use they need. Rarely does a client actually need ownership. I find this is usually my clients dealing with sensitive subject matter that is either classified, special governmental work, or other unique cases.
In the case that a client does need a buyout, I usually tack on 3 - 5x the cost of what the licensing would have been for the whole job. So if their licensing came to $5,000 total for what the pictures were potentially worth, the buyout on those would be $25,000 additional. This assumes they will use for all purposes infinitely and that you will never see another dollar from them. (hopefully that's not true...)
In terms of exclusive license, that's usually pushed upwards from the standard fee I derived, but its not nearly as far as the buyout. It really depends on the client and what kind of pockets they have too, and what the nature of the images and their use are. How valuable are these images? If they are that valuable, then they usually will pay. Others just let their jaws hit the floor and decide than an exclusive license will serve them just fine for the time frame we discussed. I'm fine with that too.
